1. Field of the Invention
The present invention relates to an assembly of welded aluminum-based elements, particularly a process for making a package of a microwave amplifier.
2. Description of the Related Art
It is well known that thermal deterioration of elements must be avoided when sealing a package for integrated circuit devices (IC's). Although a stainless steel cover and casing have been used for producing such a package, steel is not only heavy but also exhibits a relatively low thermal conductivity.
This low thermal conductivity causes a deterioration in the performance of IC's mounted in the package because of a low heat dissipation property, particularly in the package for high output amplifiers.
On the other hand, aluminum or an aluminum alloy is lighter and exhibits a desirable malleability and excellent thermal conductivity, and therefore, aluminum or an aluminum alloy is more suitable for producing a package of IC's. Nevertheless, where the aluminum or aluminum alloy elements must be bonded together to produce the above-mentioned package, this cannot be achieved by brazing, because this method makes it necessary to subject the package as a whole to a high temperature.
A local heating such as laser welding may be applied when bonding aluminum elements, but aluminum cools rapidly during laser welding because of its high thermal conductivity and the weld zone tends to crack, which leads to a nonairtight sealing of the package. Furthermore, aluminum exhibits high reflection characteristics against a laser beam. Therefore, it is impossible to fuse a thick aluminum layer by using a continuous oscillating yttrium-aluminum garnet (YAG) laser which has a relatively low output in the order of several hundred watts, and a CO.sub.2 laser. Such cracking may also occur when welding an aluminum-based alloy, such as a corrosion-resistant aluminum-magnesium alloy.
Steve Bolin et al disclose, on page 26 of an article, Laser Welding, Cutting and Drilling, Part 2, Assembly Engineering, July 1980, that the successful solution was to alter the composition of the weld joint by adding a filler metal-specifically type 4047 aluminum alloy (same as 718 braze alloy). The 12% silicon content of this material prevented the occurrence of centerline cracks by increasing the high temperature strength of the metal.